Okay, then try this.

Close the workbook (save if necessary).

Then goto File>Open and navigate the file browser to that file (don't select
it from the recent files).

Select the file from the available spreadsheets, don't click Open, but click
the drop-down arrow next to Open and select the Open and Repair option.

This will open it and repair it, fixing the lost validation. You will need
to do a saveas on the file, over the top of the previous file if you want,
as the repaired file is treated as a new file.

One more guess...

Do you use xl97 and have you used window|Freeze panes?

And are your data|validation cells above that freeze panes line?

If yes, then there's a bug in xl97 and you'll have to remove the window|freeze
panes.